In recent years, a fiber laser system in which laser beams emitted from a respective plurality of fiber lasers are combined and outputted as a multi-mode beam has been attracting attention as a way to achieve an output power of several kW (kilowatts). Such a fiber laser system is used mainly in the field of material processing.
In the above fiber laser system, there is a risk that light reflected by a processing target object, which is to be subjected to material processing, may return to the fiber laser system and cause a malfunction of it.
A reason for such malfunctioning of the fiber laser system resides in occurrence of stimulated Raman scattering (hereinafter also referred to as “SRS”) in a passage through which a laser beam propagates in the fiber laser system. The SRS can be considered as a process of power conversion from a laser beam to a Stokes beam. It is known that an increase in conversion efficiency (Raman gain) of the SRS leads to a greater likelihood of occurrence of Stokes beam oscillation. As a result, an oscillation state of each fiber laser becomes unstable and malfunctions occur in each fiber laser (see Patent Literature 1).